[opendtv] Re: DVB-T HDTV demo using 19.7 Mbps in a 6 Mhz channel in costa rica
- From: "Stephen W. Long" <longsw@xxxxxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Mon, 06 Mar 2006 19:13:55 -0500
Since Doug is so full of him self this evening, lets discuss this a little
further. These comments echo part of my discussions with the FCC years ago:
In the far field, you are going to need an antenna to receive a signal.
Since many stations tend to have antennas in the same part of town, folks
in the far field can receive all of the stations without repointing the
antenna. If you are going to all of the trouble to have a 30' antenna, the
selection of a 3db preamp to pull in a COFDM signal is no big deal. In the
far field, big antennas and preamps are the norm, and I would assert there
is no practical difference in the two systems. BTW, there are very few
viewers in the far field - at least few when it comes to ratings numbers
which is what a station cares about anyway.
The differences in the two systems are in the NEAR field - where all the
people live. COFDM is superior in the near field - the echos of buildings,
etc. make reception BETTER, and you do not have to repoint the antenna just
to change a channel. As demonstrated at NAB in 1999 you can have a
robust/mobile SD channel and an HD channel in the same bandwidth using
COFDM. More importantly, in the event of a local emergency, COFDM modes
can be switched to maximize robustness to punch through to all receivers -
to portable receivers hiding in the basement. You do not have such
flexibility with ATSC 8VSB.
With regards to the 30' antennas - what a joke. Like anyone in a city
gives a hoot about installing a 30' antenna to receive TV. I want a stub
antenna on the back of the set - just like my cell phone. I could have
this with COFDM, I will never have it with ATSC. Out in the country, it is
common to have big antennas. Given the differences in behaviors by real
consumers in the near and far fields, there is NO advantage to ATSC.
Stephen Long
At 10:51 AM 3/6/2006 -0600, you wrote:
>John Shutt wrote:
>
>>
>> What is the point of your quote? It reinforces what was actually observed
>> in the field by over a hundred different observers over the course of a
>> summer in 1999. You think you can tell the difference in 2 dB of
>> sensitivity in your living room with your receiver? Atmospherics alone
will
>> make the signal vary by more than 2 dB.
>>
>
>You BET I CAN tell a 2 dB difference, and it is large and
>obvious.
>
>Actually, of course, the difference is still more like 3 dB
>between COFDM and ATSC, but let's say 2 dB because it's
>still a big difference in reception.
>
>I have a station I watch frequently that is some 68 miles
>away. I get if off an indoor antenna, a 16 element Yagi
>sticking out a window. This is attached to a preamp
>with a 0.6 dB [sic] NF, which is is tuned for the
>channel of interest, though the 3 dB NF bandwidth is +- 4
>channels.
>
>The signal is quite reliable with this setup. I get dropouts
>for periods of a hour or so in the evening every 10 days or
>so in early winter. The signal on a spectrum analyzer is
>near perfectly flat when reception is OK. Typically the
>S/N on the receiver is in the 16.5-18 dB range. The failures
>start when it starts dropping below 16 dB into the middle
>15s. The S/N shown on the receiver meter corresponds to the
>power level shown on the spectrum analyzer as long as the
>spectrum is flat. In some reception conditions, in very cold
>winter conditions just after sunset, I start seeing a
>non-flat spectrum that varies on a 1 second time scale.
>These are very broad frequency drops, not sharp dips; as
>they vary in frequency you can see them move around inside
>this channel (which is 44) and also the same drops appear
>just as expected by extrapolation in a channel 42 station
>which is the same power, height, and on a tower about one
>mile away from 44. If the average level drops below 16 dB,
>I start seeing dropouts.
>
>Now, I have frequently tried inserting a 2 or 3 dB
>attenuator right at the input of the preamp. This has a
>DRAMATIC influence on reliability in any season except
>Aug-Oct. Instead of normally having completely reliable
>reception, it becomes quite iffy. Some days it is fine, but
>in early winter it is almost always zip, none, nothing all
>evening. In late fall or spring/early summer it becomes
>perhaps 75% reliable, which is near useless, compared to the
>normal >99.9% reliability.
>
>So I can say that IN THE ACTUAL REAL WORLD, 2 or 3 dB makes
>a HUGE difference in actual real world useability of a station.
>
>Now there is also another thing: that ch. 42 is, except in
>late summer/early fall, virtually NEVER receivable, even
>though the spectrum analyzer shows it to be exactly the same
>signal strength as 44 (and the NF of the receiver is still
>probably 1 or 1.3 dB at Ch. 42). This is because there is a
>channel 41 digital signal that is typically 40 dB stronger.
>When the propagation of 42 is good enough that the signal
>levels are less than 35 dB different, I get 42 OK.
>
>So in fact 2 dB DOES certainly make a REAL difference, at
>least in the far field.
>
>Doug McDonald
>
>
>
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